This application is a continuation in part of application Ser. No. 10/397,498, filed Mar. 26, 2003.
This invention concerns the composition, structure and fabrication of a high voltage cable that leads to and is connected to a miniature x-ray source, for applications including post-operative radiation of breast tissue and treatment within various lumens of the human body, including blood vessels.
Miniature or small x-ray tubes for human therapeutic treatment are discussed in several prior patents, including U.S. Pat. Nos. 5,854,822, 5,621,780 and 6,319,188, as well as co-pending application Ser. No. 10/397,498, commonly owned with this application. Such small x-rays tubes have been proposed or developed for the purpose of treating tumors within surgical openings in the body, for treatment within blood vessels using a catheter that contains the tube, and for other radiation treatments within the body. The cited pending application describes a cathode assembly with a cathode manufactured by MEMS technology and discloses a means of forming an extractor cup and electrically connecting the extractor cup to high voltage. The application also discloses several configurations for the high voltage cable of the device, which also carries cathode heating current on multiple inner conductors, in configurations that maximize dielectric properties to prevent arcing to a ground at an outer position on the cable. The application discloses several embodiments, and shows a form of connection of the cable to the cathode end of an x-ray tube.
The x-ray tube potential contemplated for such miniature x-ray devices is greater than 25 kV, and preferably greater than 40 kV and may be 50 kV or greater. The insulation and components in the cable, which must be quite flexible and small in diameter, preferably smaller than the x-ray tube, are required to withstand very high dielectric fields. Effective insulating material must surround and encapsulate the high voltage interior conductors, insulating them from the exterior ground. Providing enough insulating protection within a very small profile, so as to prevent arcing and cable failure, is a challenge. Placing as much insulating protection in as small a profile as possible must be achieved, while lowering the field gradient as much as possible. Such a challenge involving high voltage and extremely small size has not previously been undertaken, because the typical HV cable situation has involved much larger size or much lower voltages. Materials and design are critical, and become much more critical with reductions in size, to the order of about 1 mm external diameter, often with a requirement to pass through tight radius curves.
High voltage is divided along any path between conductors at different potentials whether or not there is a gas, solid or liquid between the conductors. The division of voltage can be proportional to the distance (linear division) or some other distribution. If the distribution is not linear, there will be a place where the voltage gradient is higher than the average linear gradient. This distribution can change with time as well as due to breakdown and material damage. When the distribution is higher than the dielectric being used can support an arc can occur.
If a solid insulator is used as the dielectric, it is normally very high resistance material. The voltage divides between the conductors based mostly on capacitance of the dielectric.
Some polymers have excellent insulating properties, rated better than glass as dielectrics. However, glass can be the ultimate insulator because it can be drawn nearly flaw free. When the glass is nearly perfect it is the optimum dielectric material for a miniature HV cable. If glass is used, sealing of the glass to the conductors is critical. In the present invention described below, glass is used as a primary insulator in several embodiments, but the use of polymers is also disclosed in several embodiments.
At the cathode end of the x-ray tube, the HV cable must be connected in a way that is rugged, that does not greatly reduce flexibility of the device so as to be capable of travel through a tight design radius, and in a way that makes effective connections of the HV conductors, including the ground, without introducing conditions that would promote arcing and breakdown.
Solutions to these problems are the subject matter of the current invention described below.